A Fabry-Perot (FP) interferometer is a multiple-beam interferometer, usually consisting of two flat plates, one of which is light transmissive and the other of which is highly reflective. The two flat plates are set parallel to one another by spacers so that light waves may bounce back and forth between them multiple times. The interferometer makes use of multiple reflections between the two closely spaced flat plate surfaces. A resonant cavity or gap of the interferometer is a region bounded by the two flat plates, which in turn is adjusted or tuned to provide multiple reflections of light waves.
Typically, a large number of interfering light waves produces an interferometer with extremely high resolution. Because of the high resolution power, the FP interferometer is widely used as a spectrometer for the accurate measurement of the hyperfine structure of spectral lines. The FP interferometer is also used as a laser resonator, since it reinforces only light of specific frequencies traveling perpendicular to the mirror surfaces. It is advantageous to fine tune the resonant cavity of the interferometer to achieve high resolution.